Distinguish MSRTT renderbuffers in the front-end glRenderbufferStorageMultisampleEXT has different semantics from glRenderbufferStorageMultisample. This change makes this differentiation in the front-end and passes a flag to the backends. This extension is currently only supported on Vulkan and D3D backends. Support for the former will be done in a follow up. Support for the latter is not planned. Bug: angleproject:4836 Change-Id: I75bc3d7990a4b1ce06264280a386d5e467983b7d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2405396 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>